Gabriela has lunch at a cafe and the cost of her meal is $\$21.00$. She would like to leave a $20\%$ tip. What is her total bill including tip?
Answer: The tip amount is equal to $20\% \times \$21.00$ We can find the tip by first calculating a $10\%$ tip and then multiplying that number by two. To calculate a $10\%$ tip, move the decimal point in $\$21.00$ one place to the left. $10\%$ $\times \$21.00$ $\$2.10$ To calculate a $20\%$ tip, multiply the $10\%$ tip amount by two. $20\%$ $\times \$21.00$ $\$2.10$ $ \times 2 = $ $\$4.20$ The cost of the meal plus the amount of the tip will equal the total bill amount. $\$21.00 + \$4.20 = $ $\$25.20$ The total cost of the bill is $\$25.20$.